Thinking, Fast and Slow by Daniel Kahneman. Kahneman takes us on a groundbreaking tour of the mind and explains the two systems that drive the way we think. Knowing how people make decisions is a key skill set for the designer in practice.
If you are even slightly interested in the Kings and Queens of England, you must read this. Of the countless books that I have read, this is the most startling and eye opening look at the way history is developed and reported that I have seen. -- to-read